Ring finals and fused spurs

[Pretty Mouth]

Evening all. The OSG says "As a rule of thumb for rings, unfused spur lengths should not exceed 1/8 the cable length from the spur to the furthest point of the ring.".

Okay, so what is the maximum length for a fused spur then? I don't seem to be able to find the answer in the OSG or BBB. I assume the limiting factor is voltage drop, but how is this determined?

 

[Charlie_]

So what’s the maximum length of a ring?

 

[Dom123]

I’m pretty sure for a 2.5mm RFC protected by a 32a is maximum of 100sqm floor space.

 

[Pretty Mouth]

OSG says for 2.5mm² on a 32A type B with RCD for disconnection times = 106m long

 

[Dom123]

That must be total length of cable then. But for sure the OSG also states an A1 RFC is maximum 100sqm floor space.

 

[Charlie_]

So based on that information how far can the furthest point of the ring be from the spur?

 

[H]

~15Meters?

 

[Charlie_]

~15Meters?

Draw the ring circuit out on a piece of paper..

 

[Pretty Mouth]

If spurred from the board, 53m

 

[Charlie_]

So based on that the maximum length would be 53/8

 

[Pretty Mouth]

I think we're driving down the wrong road here Charlie. The OSG states that 1/8th rule is for unfused spurs, I want to know the max length of fused spurs. I assume there is a difference, otherwise why would the OSG say 'unfused spurs' rather than just 'spurs'?

 

[Charlie_]

Ahh sorry didn’t realise you mentioned fused spurs..
Yes as you say it would be down to volt drop for the length of the cable and current

 

[Pretty Mouth]

That's the problem, determining the volt drop for a spur from a ring. How do you do it? Calculating the voltage drop for the spur bit I can do, but not sure about the ring part.

 

[Charlie_]

You can also do it using Zs tables and R1+R2 values..

 

[H]

You can also do it using Zs tables and R1+R2 values..

By this method do you mean
L = R x 1000 / millohmpermeter

And then Vd calculation?

Which I presume would suitably work if conductors were the same csa?

 

[Fitzy]

That must be total length of cable then. But for sure the OSG also states an A1 RFC is maximum 100sqm floor space.

For a 100sqm room you could be using any length of cable from 20m to over 100m in length, dependant on the route the cable takes etc.

 

[telectrix]

for VD on a spur, I'd estimate the length of cable from one end of the rfc and add the spur length, then use mV/A/m. this would give a pessimistic figure as the VD on the rfc part would be less.